The present invention relates to a method for operating a supercharged internal combustion engine, in particular of a motor vehicle. The invention furthermore relates to a supercharged internal combustion engine, in particular of a motor vehicle.
Exhaust gas recirculation, EGR for short, is generally known for reducing pollutant emissions and/or for reducing fuel consumption. An exhaust gas recirculation rate, EGR rate for short, here describes the proportion of the recirculated exhaust gas relative to the fresh air supplied to the combustion chambers of the internal combustion engine, the mixture of recirculated exhaust gas and supplied fresh air then forming the fresh gas or the fresh charge in the respective combustion chamber. The EGR rate in such case varies usually dependent on the load and/or speed of the internal combustion engine. In particular, when the engine is idling, or under partial load, a higher EGR rate is set than for example during full load operation.
In the case of exhaust gas recirculation, a distinction is made between external exhaust gas recirculation and internal exhaust gas recirculation. With conventional external EGR, exhaust gas is branched off from an exhaust system outside the internal combustion engine and is supplied to a fresh air system outside the internal combustion engine. The EGR thus takes place outside the internal combustion engine and to this extent externally. In contrast to this, the internal EGR takes place in the interior of the internal combustion engine. For example, after a working stroke of the respective piston of the internal combustion engine, which as a rule is configured as a piston engine, exhaust gas may remain in the respective cylinder, for example due to a certain dead volume in the cylinder at the top dead center of the piston. The proportion of exhaust remaining in the cylinder can be varied by corresponding valve closure times of the gas exchange valves. If for example the exhaust valve closes earlier than usual when the intake valve is closed, the proportion of exhaust remaining in the cylinder increases. In this case, what is called intermediate compression occurs. Further, it is possible to open the respective intake valve prematurely, i.e. earlier than usual, so that exhaust is displaced out of the respective cylinder in the direction of the fresh air system. This exhaust is drawn back in again during the following intake stroke, and thereby increases the proportion of exhaust in the fresh gas. Additionally or alternatively, it is likewise possible to close the exhaust valve with a delay, i.e. later than usual, so that exhaust gas already expelled from the cylinder can be drawn back out of the exhaust system or out of the region of the internal combustion engine which is associated with the exhaust system again during the next intake stroke. The above mentioned measures provide for an internal EGR which can be carried out alternatively or cumulatively in any combination whatsoever.
A method for operating an internal combustion engine is known from DE 10 2008 053 243 in which during a stratified charge operation the exhaust valves in the charge cycle close before a top dead center and the intake valves open after a top dead centre. This means that in the charge cycle operation exhaust gas is retained in the combustion chamber and is utilized in the stratified charge operation. This is an example of internal EGR with intermediate compression, in which exhaust gas is retained in the respective combustion chamber.
A method for operating an internal combustion engine with internal exhaust gas recirculation is known from DE 10 2009 034 763. In this case, an intake valve is opened a second time in order to force out part of the exhaust gas into the fresh gas duct during the exhaust cycle and to draw it back into the combustion chamber during an intake cycle and/or to draw exhaust gas forced out into the exhaust duct back into the combustion chamber by a second opening and closing of the exhaust valve during an intake cycle. This is an example of an internal EGR with back-suction from the fresh air side or from the exhaust side with the aid of an additional opening operation of the respective intake valve or of the respective exhaust valve.
Furthermore—as explained above—in principle it is also possible to shift the moment of closure of the respective exhaust valve and/or the moment of opening of the respective intake valve during the exhaust cycle such that early opening of the respective intake valve or late closing of the respective exhaust valve and hence the possibility of back-suction of the exhaust gas from the fresh air side or from the exhaust side is achieved.
The advantages of internal exhaust gas recirculation are considerably lesser space requirements compared with external exhaust gas recirculation. However, the cost in terms of apparatus for adjusting the EGR rate for internal EGR is considerably greater than for external EGR. Furthermore, internal EGR still has the property in principle that internally recirculated exhaust gas is at a considerably higher temperature than externally recirculated exhaust gas, since with external EGR cooling of the recirculated exhaust gas regularly takes place owing to the length of pipe, and partially also by means of an exhaust gas recirculation cooler, EGR cooler for short. This is a decisive advantage with low and medium loads, since mixture formation and ignition are supported thereby, and thus the residual exhaust gas compatibility is increased. However, with higher loads the knocking tendency increases.
Further, it has proved problematic with internal EGR that the technical devices for constituting the internal EGR (for example, a special cam form in the context of variability of the valve gear) cannot be adapted optimally to each operating point, but rather have to represent a compromise which takes account of the entire map. The result of this may be that the maximum EGR compatibility cannot be optimally exhausted at all operating points. Modern internal combustion engines are operated homogeneously lean (lambda >1) up to into the middle load and speed range, and stoichiometrically (lambda=1) in the upper load range up to full load. In the lean operating range or alternatively lean range, what is called “stratified charge operation” is used as the combustion process in the combustion chambers, in which operation a non-homogeneous distribution of the air/fuel ratio is produced in the charge of the respective combustion chamber directly before the combustion operation in the respective combustion chamber, in particular in conjunction with direct injection of the fuel. In particular, in this case provision is made, for spark-ignition internal combustion engines, to set a larger lambda value in the core region of the respective ignition means, which value decreases with increasing distance from this core region. In this respect, spatial layers with different lambda values are produced in the respective combustion chamber. For example, a rich mixture may be present in the core region, whereas a lean mixture predominates in a middle region surrounding the core region. A further, marginal, region which surrounds the middle region may even be configured to be free of fuel. In a transitional region of the lean range, between stratified and homogeneous operation, the internal combustion engine may continue to be operated lean in a homogeneous/lean operation or homogeneous/stratified operation. In homogeneous/lean operation, the internal combustion engine is operated with a homogeneous lean mixture, the homogeneous mixture being able to be ignited with the ignition means. In homogeneous/stratified operation, the combustion chambers are filled with a homogeneous lean mixture, the lean mixture present being ignited by means of a readily flammable, richer stratified charge at the respective ignition means. In particular, in the lean range with such stratified charge operation, homogeneous/lean operation and homogeneous/stratified operation, an optimally set EGR rate matters, for example in order to avoid overheating of the combustion process, which would result in increased nitrogen oxide emissions.
Such lean burn operation modes are suitable in particular for idling operation or for partial load operating conditions of the internal combustion engine. In the upper load range and full load range, the internal combustion engine is operated stoichiometrically homogeneously (lambda=1), it being possible here also to provide a cooled external EGR in order to avoid knocking. Advantageously, with the cooled external EGR the temperature in the combustion chamber can be lowered and components of the internal combustion engine can be protected from overheating, so that it is possible to dispense with enrichment of the mixture in order to control the temperature in the combustion chamber, which yields advantages in terms of fuel consumption and of emissions.
It is the object of the present invention to provide an improved operating method of the type referred to hereinbefore or for an associated internal combustion engine, which method is distinguished in particular in that the EGR rate and the temperature of a fresh charge can be adapted better to the respective operating point of the internal combustion engine.